Go inside America's largest diamond factory, which is leading a revolution in the jewelry industry

A Ritani diamond ring Sarah Jacobs New York City's Diamond District — located on 47th Street between Fifth and Sixth Avenues — glitters and sparkles, the well-lit diamonds in shop windows immediately drawing the eyes in.

But what shoppers might not realize is that behind extremely tight security and a mazes of various locked doors, diamonds are being examined, cut, and polished in the heart of Manhattan's midtown.

Online diamond retailer Ritani is one of the brands whose diamonds are perfected in a factory in the city, and when it comes to carats cut, it's the largest diamond factory in the US. It's operated by the Julius Klein Group, which is a part-owner of Ritani.

Ritani has partnered with roughly 230 local jewelers across the the US and Canada. The brand allows customers to preview their online order in a store, alongside a knowledgeable jeweler who can answer any questions.

This revolutionary "clicks-to-bricks" business model was built with the millennial male in mind. "Not only is this [engagement ring] generally their largest purchase to date, it's typically the millennial male, and he has no idea what he's doing," Ritani's Vice President of Marketing, Mark Keeney told Business Insider.

If the customer likes the ring as they view it in their local store, they can take it home that day — if not, the ring is sent back to Ritani and melted down.

Business Insider recently paid a visit to the factory to see just how Ritani's diamonds are cut and polished.

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Diamonds that come into the factory are uncut and rough. These rough yellow diamonds are about to be measured.

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To get the most value out of a rough stone, the pieces are scanned, creating a 3D version that is analyzed closely to determine if, and where, the stone should be cut before being shaped. The workers consider several different factors, including the stone's flaws and weight.

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Some stones are cut to create two separate pieces. How they cut the diamond is also determined by the market — what's selling, and what's not.

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According to Keeney, most millennials are keeping it traditional. The round and princess cut diamonds are the best sellers, although they do see micro trends, such as rose gold bands and heart-shaped diamonds, having a moment.

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The quality and price of a diamond is determined by the four C's: cut, color, clarity, and carat weight.

A yellow cut and shaped diamond
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Cut: There are various cut options to choose from when it comes to shaping a diamond, including round, princess, cushion, emerald, and more. Cut refers to both the amount of light a diamond refracts as a result of its proportions, and to the shape of the stone.

Color: This refers to the "whiteness" of a diamond. The less color, the more valuable it might be.

Clarity: Clarity is determined by how many tiny scratches are within the diamond.

Carat weight: The weight of the diamond once it's cut and polished.

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Once it's determined that a stone needs to be cut, it's sent to the laser cutter, which creates a precise cut. This process can take up to three hours, and is technically difficult to pull off correctly. The man behind all of Ritani's diamonds has studied continuously for over five years with the manufacturer of the machine in Antwerp, and in the US.

Inside the laser cutter
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Next, the cut diamonds go a work station. Here, a craftsman who specializes in specific diamond cuts works with the stone to create a masterpiece.

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Many of the craftsmen have been working at this factory for 40 years or more.

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Diamonds are placed within the "tang," a device that holds the diamond as it's being polished.

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The tang is then placed above a spinning disc that is coated in diamond dust, which cuts the stone.

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The craftsman either places weight with his hands or uses block weights to push the diamond in the tang onto the spinning diamond dust, creating the facets of the diamond that make it sparkle.

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Diamond cutting can range from a few weeks to several months, based on the size, cut, and overall polishing process. One round diamond can have up to 58 facets, creating what Ritani calls "maximum sparkle."

A craftsman inspects a diamond.
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The largest, most precious rare diamonds can take up to a year to cut.

Yellow diamond
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After cutting and polishing is done, the diamond makes its way to these machines, which polish the girdle of the diamond and ensure uniform roundness.

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After the final polish, the diamond is thoroughly examined to GIA (Gemological Institute of America) standards before being independently certified.

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The stone is then ready to be set in a ring.

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Ritani's online shopping aspect extends into the store. IPads on the counter tell you the exact specs of the ring you've ordered.

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An Apple TV is also provided so that customers can compare the diamonds' size, cut, clarity and price to make a final decision.

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Recently, Ritani sold a three-stone ring with three individual extremely rare colored stones for close to $3 million.